Recently, energy savings, decrease in the manufacturing cost and improvement in quality of products have been required increasingly. Also in the field of grinding of cam profiles, automobile manufacturers have played a most active part in requesting shortening of cycle time and improvement in machining accuracy. However, the two requirements are incompatible with each other. Therefore, in spite of various attempts at satisfying both requirements, no satisfactory result has been obtained yet.
In cam profile grinding, a rotary motion and a rocking motion conforming to the profile of a master cam are imparted to a workpiece, and the rate at which the workpiece is removed by grinding, that is, angular displacement d.theta. per unit time varies constantly, as shown in FIG. 1. This quantity of change becomes larger if the workpiece is rotated at higher velocity for a constant time of grinding, that is, a constant removed quantity per unit time. At the same time, it is more likely that vibration occurs, but less heat, grinding burn and cracks are produced because the arcuate length l.sub.a of the workpiece in contact with a grinding wheel decreases as illustrated in FIG. 2 (A). In the prior art cam grinding making use of this characteristic, a workpiece is rotated at a high velocity in the order of 80 rpm over a rough grinding cycle, but the infeed velocity F.sub.1 of the grinding wheel higher than about 25 mm/min is not used, because if the infeed velocity exceeds this value, great vibrations and a large uncut portion are introduced. Thus, it is quite difficult to increase the machining efficiency further. The rough and fine grinding cycles in this case are shown in FIGS. 3 and 4, respectively, in which N.sub.1 and N.sub.2 indicate high and low velocity rotation regions, respectively, F.sub.2 is the infeed velocity at finishing that is set to about one-tenth the velocity F.sub.1, and D.sub.1 and D.sub.2 are allowances for rough and finish grindings, respectively. These allowances are so set that the relation D.sub.1 =15 D.sub.2 holds.
On the other hand, when the workpiece is rotated at a lower velocity, the quantity of change of the grinding removal rate is smaller and the arcuate length l.sub.b of the workpiece in contact with the grinding wheel is longer as illustrated in FIG. 2 (B). Accordingly, the load imposed on each one abrasive grain is lighter and the acceleration that a rocking table experiences is smaller, permitting an increase in the infeed velocity of the grinding wheel. The prior art cam grinding utilizing this characteristic is effected under such conditions that the workpiece is rotated at a low velocity of 30 rpm (N.sub.3) when the grinding wheel is pressed against the workpiece and that it is rotated at a high velocity of 60 rpm (N.sub.4) during spark out occuring after the cutting. In such a grinding operation, the infeed velocity of the grinding wheel can be made larger than the foregoing value and can be increased to about 40 mm/min (F.sub.3), but the slow velocity of the rotation of the workpiece increases the arcuate length l.sub.b in contact with the wheel as shown in FIG. 2 (B), whereby grinding burn and cracks occur more often. For this reason, the grinding velocity is unwillingly made low, sacrificing machining efficiency. The rough and fine grinding cycles are illustrated in FIGS. 5 and 6, respectively, where the infeed velocity F.sub.4 at finishing is set to be about one-tenth the velocity F.sub.3. The values of the allowances D.sub.1 and D.sub.2 for rough and fine grindings, respectively, are set so as to be substantially the same as those in FIGS. 3 and 4.